Let z = x²,dz = 2x dx
∫ x√[(1 - x²)/(1 + x²)] dx
= (1/2)∫ √(1 - z)/(1 + z) dz
= (1/2)∫ √(1 - z)/√(1 + z) · √(1 - z)/√(1 - z) dz
= (1/2)∫ (1 - z)/√(1 - z²) dz
= (1/2)∫ dz/√(1 - z²) dz - (1/2)∫ z/√(1 - z²) dz
= (1/2)∫ dz/√(1 - z²) dz - (1/2)(- 1/2)∫ 1/√(1 - z²) d(1 - z²)
= 1/2 · arcsin(z) + 1/4 · 2√(1 - z²) + C
= (1/2)arcsin(z) + (1/2)√(1 - z²) + C
= (1/2)[arcsin(x²) + √(1 - x⁴)] + C